Generally, rubber compositions used for rubber articles such as tires are prepared by blending rubber components with various compounding agents such as fillers and vulcanizing agents and kneading them. Here, in order to improve workability in the kneading of a mixture prepared by adding compounding agents such as fillers to rubber components, oil components such as process oil are usually added to the mixture. Further, attempts have been made to improve physical properties of the rubber composition, by using liquid polymers as such oil components or using low molecular weight polymers as an alternative to the oil components.
For example, WO 2005/087858 A (PTL 1) discloses that a rubber composition having a high storage elastic modulus and a low loss factor while having excellent fracture resistance can be obtained without impairing the workability of the rubber composition, by adding an aromatic vinyl compound-diene compound copolymer having a weight-average molecular weight of 5,000 to 300,000 instead of the oil component. Further, in WO 2005/087858 A, from the viewpoint of reducing the loss factor of the rubber composition, and in order to obtain an aromatic vinyl compound-diene compound copolymer with a relatively narrow molecular weight distribution, an aromatic vinyl compound-diene compound copolymer is synthesized by solution polymerization using an organic lithium compound as an initiator. The aromatic vinyl compound-diene compound copolymer thus synthesized is used in the rubber composition.